1. Field of the Invention
The present invention relates to a wireless communication system, and more particularly, to a method and apparatus for controlling transmission power in a wireless communication system using Fractional Frequency Reuse (FFR).
2. Discussion of the Related Art
A multi-carrier Orthogonal Frequency Division Multiple Access (OFDMA) system allocates resources in units of subchannels each including subcarriers. Since multiple users share total subcarriers, multi-user diversity gain can be achieved in the frequency domain. An OFDMA-based broadband mobile Internet access system such as a Wireless Broadband (WiBro) system can maximize throughput by reusing the same frequency in all cells and applying Adaptive Modulation and Coding (AMC) according to received signal strengths and inter-cell interference.
A geographical coverage unit is called a cell or sector and frequency switching between cells to continue an on-going call is called handoff. Frequency reuse is essential to a cellular system. A frequency reuse factor is calculated by dividing the number of cells or sectors using the same frequency simultaneously by the total number of cells in a multi-cell structure.
A 1st Generation (1G) system (e.g. Advanced Mobile Phone Service (AMPS)) has a frequency reuse factor smaller than 1. For example, the frequency reuse factor is 1/7 in 7-cell frequency reuse. The frequency reuse factor is higher in a 2nd Generation (2G) system (e.g. Code Division Multiple Access (CDMA) and Time Division Multiple Access (TDMA)) than in the 1G system. For instance, a Global System for Mobile communications (GSM) system operating Frequency Division Multiple Access (FDMA) and TDMA in combination boasts a frequency reuse factor of up to 4 or 3. A 2G CDMA or 3rd generation (3G) Wideband CDMA (WCDMA) system may support a frequency reuse factor of 1, thus increasing spectral efficiency and reducing network deployment cost.
The frequency reuse factor of 1 can be achieved when all sectors within a cell and all cells within a network operate on the same frequency channel. Nonetheless, even a system with the frequency reuse factor of 1 may suffer from poor throughput at a cell edge or sector edge due to severe interference between neighbor cells and thus may face service outage. That is, signal reception performance is poor for users at a cell edge because of inter-cell interference.
In OFDMA, a channel is divided into subchannels and a signal is transmitted on subchannels. Unlike 3G (CDMA2000 or WCDMA), an entire channel is not occupied for signal transmission. Throughput may be increased at the same time for users at a cell center and users at a cell edge by taking advantage of this feature.
To be more specific, a cell center is an area close to a Base Station (BS) that is relatively immune to co-channel interference. Thus users at the cell center may operate on all available subchannels. On the other hand, users at a cell edge are only allowed to operate on a fraction of all available subchannels. Fractions of sub-channels are allocated in such a way that neighbor cells' edges will operate on different subchannels. This is called FFR. The co-channel interference between neighbor cells can be mitigated by orthogonally dividing total subcarriers into a plurality of Frequency Partitions (FPs) and deploying the FAs such that each cell does not use a certain FA or uses the certain FA at a low power level.